Raw laser beams may be produced from a variety of mechanisms. It is known in the art that raw laser beams need to be temporally profiled in order to have proper applications in areas such as laser communications, laser signal processing, laser spectroscopy, laser inertial confinement fusion, etc. Typically, laser pulse shaping in the time or frequency domain is used for the temporal profiling of very short input laser pulses. In the case of longer pulses, the direct modulation of beam intensity using a Pockels cell (that alters the polarization state of light passing through it when an applied voltage induces birefringence changes in an electro-optic crystal) is employed.
In particular, a simple approach for temporal profiling of laser pulses includes the direct modulation of pulse intensity using a Pockels cell placed between two polarizing elements. In this approach, application of a temporally profiled electrical pulse to the Pockels cell results in a time-dependent attenuation of the input laser pulse. One example of a device exploiting such an approach is disclosed in U.S. Pat. No. 6,421,390, which is incorporated by reference herein in its entirety. A basic disadvantage of this approach is that it has limited capability to form optical pulses with special temporal profiles, especially with temporal profiles having relatively fast rising optical intensity.
Laser beam pulse shaping in the time domain can also be based on an approach that first temporally broadens a short laser pulse using a fiber, temporally profiles the broadened chirped pulse, and then compresses it using a grating pair to synthesize the desired pulse shape. An example of this approach is disclosed in M. Haner, W. S Warren. “Synthesis of crafted optical pulses by time domain modulation in a fiber-grating compressor,” Appl. Phys. Lett., v. 52(18), 1458-1460, 1988, which is incorporated by reference herein in its entirety. This approach is used for generating voltage programmable and arbitrarily shaped laser pulses in subpicosecond durations. The disadvantage of this approach is that it is a three-stage process. The first stage is the creation of the chirped pulse. Then, this pulse is shaped by programmable microwave generator. Finally, it is compressed using a grating pair to synthesize the desired pulse shape. As such, this approach requires the use of complicated and expensive equipment (e.g., the microwave generator) to create the proper and accurate temporal shape of the desired laser pulse.
A. M. Weiner, J. P. Heritage, E. M. Kirschner. “High-resolution femtosecond pulse shaping,” JOSA B, v. 5, 1563-1572, 1988, which is incorporated by reference herein in its entirety, discloses an approach based on spatially separating spectral components of a laser pulse and modifying the amplitudes and/or the phases of these components to produce the optical spectrum corresponding to the desired temporal pulse profile. Then, the spectral components are recombined to obtain the desired temporal shape of laser pulse. This approach can only be used to generate simple optical pulse profiles because it requires a very detailed and accurate modification of the amplitude-phase relation between the spectral components.
An additional disadvantage in each of the above-mentioned approaches is that these approaches result in large losses in total energy due to attenuation of the input pulses.
U.S. Pat. No. 3,826,561, which is incorporated by reference herein in its entirety, describes an approach for time-tailoring the intensity of focused laser beam pulse reflected from a reflector onto a focal region. This approach permits stepwise or continuous shaping in space and time which is accomplished by dividing up and reassembling portions of the laser beam. This approach has a basic disadvantage that it only allows temporal profiling of pulse intensity in the focal region. In addition, this approach has the further disadvantage that the different temporal parts of the pulse irradiate the focal region from different directions and that it requires complicated and expensive equipment to control and realize the needed temporal beam intensity.
In view of the above, it is desirable to provide a system and method for temporal shaping or profiling of laser beams that overcomes the above-described shortcomings while retaining their advantages. In particular, it would be desirable to provide a system and method that can realize practically any temporal output beam shape and/or any distribution of the beam intensity in time (e.g., longer than the input pulse duration) without requiring complicated controlling equipment, without big losses in beam power, and/or without requiring a special distribution of the intensity of the initial (incident, raw, or input) laser beam.